Presently known immersion freezers comprise a perforated conveyor which is, along at least part of its length, dipped into a bath of liquid refrigerant, such as liquid nitrogen. The liquid nitrogen acts to freeze the product before it is removed from the bath. In order to enable the conveyor to be snaked into and out of the liquid nitrogen and to ensure that the residence time of the product under the surface of the liquid nitrogen is sufficient to avoid underchilling, the bath must be somewhat larger than desired. It will be appreciated that the larger the bath the greater the volume of liquid nitrogen and hence the greater the waste thereof. Additionally, handling large quantities of liquid nitrogen safely can be a problem. Furthermore, the conveyor can become blocked with ice and/or small particles of food product and must be removed from the liquid nitrogen for cleaning. Clearly, it is impossible to continue operating the freezer while the conveyor is being cleaned.
An alternative immersion freezer is provided with a continuous belt which passes in one direction through a bath of liquid nitrogen and in the other direction beneath the bath itself, thereby avoiding further immersion in the liquid nitrogen. In this arrangement, it is also known to shape the bath such that the belt drops vertically into the front of the bath and is raised gradually from the rear thereof. Such an arrangement reduces the overall bath volume and hence reduces slightly the problems of liquid waste and safe handling. However, it is obviously impossible to lift the belt out of the liquid nitrogen for cleaning without discontinuing the freezing operation.
An additional problem associated with both of the above mentioned arrangements resides in undesirable liquid `boil off` which occurs due to the cyclic cooling and heating of the conveyor as it passes into and out of the liquid nitrogen.